This invention pertains to a process for the preparation of porous or cellular resinous bodies.
Previously, U.S. Pat. No. 2,450,436 issued Oct. 5, 1948 to O. R. McIntire disclosed the method of producing a plastic cellular article by charging a vessel with a solid thermoplastic polymer, e.g. polystyrene, together with a normally gaseous agent capable of being dissolved by the polymer, subjecting the mixture to superatmospheric pressure--to form a gel of the polymer--and opening an outlet--so as to cause the gel to flow from the vessel. The invention is restricted to release of the pressure on the solution at a time when the latter is at a temperature preferably between 50.degree. and 125.degree.C.
The use of certain specific nucleating agents in controlling pore size of polystyrene foamed articles is claimed in U.S. Pat. No. 3,188,295 issued June 8, 1965 to D. E. Ballast et al, wherein the organic pigments indigotin and blue copper phthalocyanine are shown. Furthermore, in U.S. Pat. No. 3,224,984 a relatively low molecular weight (1000-4000) polyethylene wax is used as nucleating agent.
A combination of nucleating agents, wetting agents and a foaming agent was taught in U.S. Pat. No. 3,413,387 to E. O. Ohsol, but his nucleating agents were made up of two materials which react to form carbon dioxide and water, and the foaming agent was absorbed on an absorbent. The use of polystyrene as a nucleating agent for foamed polyethylene was taught in U.S. Pat. No. 3,065,190 to D. S. Chisholm et al.
In Cellular Plastics, 4, No. 9, September 1968, "Evaluation of Four Foaming Techniques for Inflating a PK-Life Raft", by I. O. Salyer, J. L. Schwendeman and C. E. McClung, compositions are described similar to those of the invention, but without the critical feature of the particular nucleating agents required for certain polymers to produce foams of small and uniform cell size.
Despite the advances made in the prior art, we have found that the prior art methods yielded foams of widely varying cell size, usually with cells larger than desired, and consequently, of relatively low compressive strength. Additionally, the foams frequently shrink, or even collapse, or the blowing agent volatilizes before the foam hardens sufficiently to support its own weight.
Furthermore, the use of organic pigments as nucleating agents yielded colored products which are limited in utility.
Last, but not least, the products of this invention can be prepared at room temperature, and do not require the elevated temperature required by the prior art. Consequently, these foams can be generated in areas remote from complex processing equipment, and require only a valved pressure vessel for containment until ready for use. The foam can even be discharged incrementally from the pressure vessel with prolonged periods of time between each release.
The products of the present invention are useful for flotation devices, for packaging of delicate equipment, for emergency splints for personal injuries, for thermal insulation, for closing openings, for decorative purposes, etc. The foam is generally low-density with predominately closed-cell structure having high compressive strength and dimensional stability, with small uniform cells. Its whiteness makes it attractive for aesthetic reasons. However, it may be dyed readily, as by adding a soluble dye such as Congo Red or Brom Cresol Green in suitable concentration to the foamable mixture, e.g. 0.2-2.0% by weight of the solid foam.